The increase in fluctuating power production requires an increase in flexibility in the system as well. Flexibility can be found in generation technologies with fast response times or in storage options. In the transport sector, the proportion of electric drive vehicles is expected to increase over the next decade or two. These vehicles can provide some of the flexibility needed in the power system, in terms of both flexible demand and electricity storage. However, what are the batteries worth to the power system? And does the value depend on battery capacity? This article presents an analysis of the integrated power and transport system, focusing on the sensitivity of the power system configuration according to battery capacity and price of the electric drive vehicle. The value of different battery capacities is estimated, given that the batteries are used for both driving and storage. Likewise, the prices at which the electric drive vehicles become of interest to the power system are found. Smart charge, including the opportunity to discharge (vehicle-to-grid) is used in all scenarios. Analyses show that the marginal benefits decrease the larger the battery. For very high battery prices, large batteries imply that diesel vehicles are preferable to electric drive vehicles.